Steam cracking appeared as early as 1920 for producing ethylene from ethane. It became very soon a basic process in petrochemistry for treating heavier and heavier charges which may be as heavy as vacuum gas-oils.
It is based on the relative instability at high temperature of paraffins and naphthenes as compared with olefins and aromatics. The main reactions involved are the breaking of a C-C bond by a homolytic breaking mechanism, giving an olefin and a paraffin, and dehydrogenation. Both reactions are endothermic and hence favored by a temperature increase. They also result in an increase of the number of molecules present and they are accordingly favored by low partial pressures of hydrocarbons to be treated. For this reason said pressure is reduced to a maximum extent by adding steam to the reaction medium.
However, it has been observed that, when a hydrocarbon charge is maintained at a temperature higher than 800.degree. C. for about a few tenths of second, coke deposits are quickly formed, resulting in several disadvantages: decrease of the heat transfer between the reactor and the charge to be cracked, substantial increase of the reactor wall temperature, and a decrease of the useful sectional area of the reactor resulting in an increase of the pressure drop inside the reactor, making necessary the shut down of the unit for coke removal.
Coke formation is due to secondary reactions such as the formation of condensed polycyclic aromatic hydrocarbons, as well as to the polymerization of the olefins formed.
This last reaction results from the tendency of olefins to polymerize at a temperature of about 500-600.degree. C. It is thus advisable, for decreasing the extent of said secondary reaction, to subject the reaction effluents to a quick cooling (often called "quenching") so as to bring them quickly from the pyrolysis temperature to a temperature lower than 500.degree. C., generally by indirect heat exchange means.
It has been further observed that the olefin polymerization was favored by the presence, at the surface of the heat exchanger metal walls, of nickel which acts as polymerization heterogeneous catalyst (M. DENTE and coll., "Fouling of transferline exchangers in ethylene plants", AICHE Meeting of Houston, Tex., Mar. 30, 1983).
Thermodynamic and kinetic surveys of hydrocarbon pyrolysis reactions have thus indicated that the selectivity to olefin production can be increased by the following operations:
quick temperature increase of the charge up to the optimum pyrolysis temperature for a given charge, said temperature being maintained as constant as possible in the reaction zone,
decrease of the charge residence time in the reaction part,
decrease of the hydrocarbon charge partial pressure,
quick and efficient quenching of the reaction effluents for avoiding secondary reactions of the type of olefin polymerization.
Technologically, these required operations are conveniently conducted according to a general process comprising the steps of:
a) preheating the charge, diluted with steam,
b) heating this mixture at high temperature in tubular furnaces in order to limit the hydrocarbon residence time during this pyrolysis phase,
c) quickly quenching the reaction effluents.
The technology development essentially concerned the pyrolysis step (b) and the quenching step (c), in order to fulfill the above-mentioned requirements and to treat a variety of charges presently extending from ethane to vacuum gas-oils.
The steam cracking furnaces were improved essentially for reducing the residence time and the pressure drop, by decreasing the length of the tubular reactors, thus increasing the thermal flow, particularly near the reactor feeding port.
Moreover, for reducing corrosion phenomena to a minimum, the furnaces must be heated by means of fuels of high quality, of low sulfur content, such for example as natural gas or fuel-gas produced by steam-cracking itself, thus increasing the operating cost of the process.
With respect to the quenching of the effluent reaction products, the technology was oriented towards heat exchangers provided with transfer lines for the pyrolysis reaction effluents (called TLX exchangers, "transfer line exchangers" disclosed for example in U.S. Pat. No. 4,097,544).
The object of these exchangers is to produce, as quickly as possible, a sharp decrease of the pyrolysis reactor gas effluents at such temperatures that secondary reactions of olefin polymerization type do not occur.
However, the temperature of the output effluent of the quench exchanger varies in accordance with the steam-cracked charge. For example, when treating vacuum gas-oils of aromatic type, a relatively high amount of condensed polyaromatic fuel-oils, present in the steam-cracking effluents, cannot be quenched at low temperature without giving rise to excessive clogging of the exchanger, liable to reduce the operating time of the furnace. Here, two-step cooling is generally preferred, the first step being performed by indirect quenching in the quench exchanger down to a temperature of about 450-500.degree. C. and the second step consisting of direct cooling by introduction of cold liquids in the exchanger effluents.
An attempt was made by manufacturers of "TLX" type exchangers for reducing the dead volume between the tubular reactor outputs and the effluent inputs in the quench exchanger (without complete success), which is detrimental to a quick quenching. In addition, these exchangers are built of refractory steel containing nickel which is an olefin polymerization catalyst.
None of the known techniques for hydrocarbons steam cracking is, to the best of our knowledge, fully satisfactory. In particular, even with the use of special alloys such for example as INCOLOY 800 H for the tubular furnaces, it is not possible to exceed a reaction temperature of about 1,100.degree. C. and hence to quickly bring the charges to temperatures at which thermal cracking is performed in good conditions. Moreover, it is known that the maximum heat amount must be supplied to the tubular reactor zone where the C--C bond cracking and dehydrogenation drogenation endothermic reactions take place, but this is not satisfactorily achieved in the known processes.
On the other hand, the requirement of maintaining high thermal flow has been met by reducing the cross-section of the pyrolysis tubes, making a decrease of their length in order to maintain an acceptable pressure drop. Moreover, none of the present processes is operable with a substantially constant temperature all along the reaction zone. In addition, the heterogeneity of the thermal flow results in substantial temperature differences at the outer surface of each tube.
Steam cracking processes of the prior art, not operated in tubular reactors, are disclosed for example in U.S. Pat. No. 4,411,769 where, in an integrated ,coking and steam cracking process, it is suggested to use very hot coke particles of sufficient temperature to supply the heat necessary for the endothermic steam cracking reaction.
The prior art is also illustrated by U.S. Pat. No. 4,370,303 which discloses a steam cracking process where the charge is cracked in the presence of steam by contact with hot solid particles in a first portion of a reaction zone, the obtained effluent is then separated from the hot particles in a primary separation zone and then in a separation zone of cyclone type.
It is then subjected to quenching by contact with cold particles in a second portion of the reaction zone separate from the first one.
But the step of contacting hot particles with the charge, the primary separation step and the cyclone separation step are performed in different enclosures, thus resulting in increased engineering cost and in substantial lengthening of the total residence time of the effluent before cooling, independently from that concerning the passages from one enclosure to another through the transfer lines.
The technological background is further illustrated by European patents 154,385 and 226,483, British patent 709,583, U.S. Pat. No. 2,846,360 and French patent 2, 576,546.